Preserve multiplicity throughout Species representations

[alongd]

When an xyz guess is given for a species, the .mol and .mol_list attributes are updated accordingly to match atom order. However, currently the multiplicity is guessed by openbable, and it might not be the same as the multiplicity given via the 2D graph representation.

A similar issue occurs when testing for isomorphism after generating conformers.

This PR makes sure the species multiplicity is consistent throughout its representations.

[codecov]

Codecov Report

Merging #99 into master will increase coverage by 0.07%.
The diff coverage is 52.17%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   41.19%   41.27%   +0.07%     
==========================================
  Files          22       22              
  Lines        5042     5098      +56     
  Branches     1303     1324      +21     
==========================================
+ Hits         2077     2104      +27     
- Misses       2634     2655      +21     
- Partials      331      339       +8

Impacted Files	Coverage Δ
arc/species/species.py	58.56% <100%> (+0.11%)	⬆️
arc/scheduler.py	20.05% <20%> (-0.1%)	⬇️
arc/species/converter.py	75.22% <50%> (-6.24%)	⬇️
arc/rmgdb.py	73.33% <66.66%> (-0.64%)	⬇️
arc/processor.py	58.62% <75%> (+0.39%)	⬆️
arc/reaction.py	42.12% <0%> (ø)	⬆️

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